Non-Sequel =/= new IP
I wholeheartedly prefer a smaller selection of IPs than a huge cluster of new IPs every year.
Nintendo always try to look into new ways for players to play and experience their games, existing and new IPs alike. They're very likely working with gameplay ideas way before even considering what universe it will take place in. If they think their current IPs won't cut it, then a new one will be in order that fulfills the criteria for their new game.
There's a reason why Mario, Zelda, etc are doing so well. They're in fact most of the time not direct sequels but new experiences in the same universe. IPs that often deilver a more lasting impact on players due their larger history and character development. Something i prefer over short term IPs where the experience in due time will fade away, Nintendo is one of the few game companies that actually succeed with releasing games within their current IPs. They may have problems with direct sequels as well but absolutely not to the same extent you implied.
If gameplay is new and innovative enough within its boundaries, it shouldn't be necessary to create a new IP to make a game feel new.